1. Field of the Invention
The present invention is directed to an improvement in olefin polymerization in reactor systems employing heterogeneous single site catalysts supported on a porous inorganic support and employing an activator.
2. Background Art
Numerous processes exist for the preparation of polyolefins. The various processes may be divided into solution polymerization processes employing homogeneous (soluble) catalysts, and processes employing supported (heterogeneous) catalysts. The latter processes include both slurry and gas phase processes. All these processes are well known to those skilled in the art. The subject invention is directed to slurry and gas phase processes.
Prior olefin polymerization processes generally utilized the so-called Ziegler-Natta catalyst systems. In such systems, a variety of active transition metal compounds, many based on titanium and/or chromium compounds, employed trialkylaluminum compounds as co-catalysts. These catalyst systems are still used in large quantities today.
More recently, a variety of organometallic olefin polymerization catalysts have been developed. These are often termed “single site” catalysts because the polymerization is thought by some to occur at a single site on a complexed transition metal center. These catalysts have proven to be relatively inactive when alkylaluminums are employed as co-catalysts, but highly active in the presence of alumoxanes, particularly methylalumoxane, or in conjunction with a bulky, non-coordinating anion such as the tetrakis(pentafluorophenyl)borate anion. Many of these catalysts are bis, π-complexes of cyclopentadiene, i.e. “metallocenes,” the simplest of which include bis(cyclopentadienyl)zirconium dichloride. Complexes containing but a single π-bonded cyclopentadienyl moiety, or three of such moieties may also be useful. These latter are sometimes termed “metallocene” catalysts herein, although the term “metallocene” conventionally applies only to bis(cyclopentadienyl) complexes. The cyclopentadienes may be substituted or unsubstituted, and may be linked through a variety of bridging groups. Examples of such catalysts may be found in numerous references, including U.S. Pat. Nos. 5,064,802; 5,198,401; 5,408,017; 5,504,049; 5,599,761; 5,663,249; 6,232,630; 6,232,260, and 6,376,629 incorporated by reference herein. In addition to the catalysts described above, a variety of multidentate metal complexes have been found to be effective olefin polymerization catalysts. Numerous examples may be found in the patent and non-patent literature, including the quinolinoxy catalysts described in U.S. Pat. No. 5,637,660. The term “organometallic catalyst” will be used herein for such catalysts. Because of their unique catalytic activity, metallocene catalysts are considered different from Ziegler-Natta catalysts by those skilled in the art, and polymerization processes and additives employed therein are ordinarily modified to take into account their differing properties.
The organometallic catalysts used as heterogeneous catalysts are supplied to the reactor on a porous support material such as porous alumina or silica, the latter being highly preferred. It has been found that by depositing the co-catalyst or activator (both terms are used somewhat interchangeably) onto the support as well, highly efficient supported catalysts can be obtained. The catalyst and activator can be deposited in many different ways, including both different orders of addition as well as in different modes of addition. Prereacted products of single site catalyst complexes and activator can also be deposited. It has been found that the various different deposition processes can result in unexpected differences in catalytic activity in some cases. Examples of supported catalyst preparation include U.S. Pat. Nos. 5,006,500; 5,468,702; 5,863,853; 5,240,894; 5,554,704; 5,635,437; 5,416,178; and 6,172,168, which are herein incorporated by reference.
Particularly in gas phase polymerizations, sheeting phenomena may occur which result in difficulties in maintaining continuous operation. Although numerous theories have been proposed for such phenomena, it is generally understood that polymer particles may adhere to the reactor walls, particularly at points of low polymer particle velocity. The adhering particles generate heat by continued polymerization, and also serve to thermally insulate the reactor wall, preventing efficient cooling. These “hot spots” may rise to a temperature above the melt temperature of the polyolefin, causing the particles to melt together to form clumps, agglomerates, and sheets. These fused or partially fused artifacts may slough off the reactor walls and then tend to block takeoff lines for polymer particle product. They also must, in general, be removed from the polymer particle product. One proposed method for preventing such sheeting phenomena is to deactivate small catalyst/polymer particles by adding glycols, glycol ethers, or sorbitan monooleate, as disclosed in EP0560035A1. A further method, employing fatty amines deposited on the supported catalyst, is disclosed in U.S. Pat. No. 6,201,076.
Adherence to the reactor walls may occur due to an inherent tackiness of the polymer particles, or may be induced by electrostatic attraction. As the essentially non-conductive polyolefin particles are circulated in the gas phase reactor, electrostatic charges build up, as can be shown, in some cases, by appropriate sensors. The static build-up in gas phase reactors is most pronounced in the lower (up to 1 bed diameter) portion of the reactor, and it is here, also in conjunction with less intense mixing of particles near the wall, that the majority of hot spots and associated sheeting phenomena occur. Reference may be had to U.S. Pat. No. 4,792,592 in this regard, and to U.S. Pat. No. 5,283,278, where “antistats” such as chromium salts of C14-18 alkylsalicylic acids are added to the polymerization reactor in Ziegler-Natta catalyzed olefin polymerization.
Generation of hot spots, sheeting, and static buildup have all been the subject of much discussion in olefin polymerization. However, the interrelationship among such phenomena is still not well understood. Moreover, the addition of “antistats” to the polymerization produces quite variable results, and frequently is accompanied by a loss in catalyst activity and/or impairment of polymer physicochemical properties such as polymer particle morphology and bulk density. For example, U.S. Pat. No. 6,140,432 discloses adding a primary, secondary, or tertiary hydroxyalkylamine to a supported catalyst. Such compounds can seriously impair catalyst activity.
In slurry polymerization processes, static buildup is not ordinarily a problem. Moreover, particle velocity is generally high due to continued and rapid circulation of the slurry in the reactor when slurry loop reactors are employed. Antistat-treated catalysts have been proposed for use in such reactors nevertheless, as sheeting problems can still occur. See, e.g., U.S. Pat. No. 6,201,076. However, equally detrimental to the slurry process is a decrease in polymer bulk density exhibited by such catalysts. Lower bulk density can adversely affect harvesting of the polymer from the reactor, i.e. by the use of settling legs or other means which rely on gravitational sedimentation of the polymer from the liquid continuous phase.
It would be desirable to provide a process of olefin polymerization employing supported catalysts wherein the overall polymerization process is improved with respect to the problems discussed above, without incurring the penalty of decreased catalyst activity or production of polymer particles having less optimal physicochemical properties.